Hamadi Jebeli, secretary-general of Ennahda, the moderate Islamist party which romped to victory in last month's elections, told a rally in the city of Sousse: "My brothers, you are at a historic moment in a new cycle of civilisation, God willing. We are in sixth caliphate, God willing."

Party officials, who have spent months insisting they wanted to pursue secular democratic politics rather than an international Islamist agenda, were forced on the defensive after his comments were posted on the internet.

But they may have scuppered the party's hopes of forging a broad-based coalition. The left-of-centre secular Ettakatol, which came third in the vote, suspended talks with Ennahda over forming a government.

"We thought we were going to build a second republic with our partner – not a sixth caliphate," Khemais Ksila, a senior member, said.

Mr Jebeli was imprisoned under the dictatorship of Zine al-Abedine Ben Ali for his Islamist beliefs. But since the country's revolution in January, the party has argued it wants to work within a democratic political system along western lines.

The stance was enough to see them win 40 per cent of seats in parliament, more even than their supporters expected. But opponents say their moderate public statements hide a more radical agenda – one more honestly expressed to provincial working class supporters than the middle classes in the capital.

Ennahda's other prospective coalition partner, the likewise centre-left and secular Congress for the Republic, did not follow Ettakatol's lead, perhaps soothed by a deal making its leader Moncef Marzouki interim president.

The term "caliphate" refers to a governance system based on sharia law as formerly used by successive Islamic empires. It is a sensitive term in Arab politics due to its modern associations with extremist groups like al-Qaeda and especially in Tunisia where it is promoted by the radical Salafi movement, Hizb-ut-Tahrir.

Mr Jebeli's opponents are now planning a large demonstration to coincide with the opening of the new assembly.